Glasses are generally colored by incorporating three types of elements:                i) coloring ions (iron, manganese, chromium, etc.);        ii) nonmetallic centers (selenium, phosphorus, etc.) or certain compounds thereof; and        iii) metal atoms (gold, silver, copper).        
These elements thus give the glasses into which they are incorporated certain properties of absorbing incident radiative energy. When incident radiation is absorbed, the material changes to an excited state. In general, it returns to its initial (i.e. unexcited) state by dissipating the surplus energy in the form of heat. In certain cases, it may also dissipate this energy in the form of light radiation of lower energy than that having resulted in the excited state: this is luminescence (comprising fluorescence, in which the lifetime of the excited state is very short; and phosphorescence, in which the lifetime of the excited state is relatively long). The lifetime of the excited state may be influenced by the composition of the glass. In the case of uranium, the lifetime is short in an alkaline glass (the glass is therefore fluorescent) and long in a glass having a high silica content (the glass may then instead be phosphorescent).
Several factors may influence the luminescence intensity of materials containing active centers. For example, in general the luminescence yield of a glass containing a luminescent center is lower than for a crystalline material containing this same active center. Other factors affecting the luminescence that may be mentioned include the temperature and attenuation or extinction (more commonly known as “quenching” effects, by concentration of the fluorescent center). In the case of glass, the chemical composition of the latter may also limit the luminescence: iron is the main impurity that may reduce or even completely quench the luminescence. Certain halogens have the same effect in the glass.
Among all the active centers that may make a glass fluorescent, rare-earth ions constitute a special class. Their specificity stems from the way in which their electrons occupy the various energy levels. This is because they possess a full outer shell protecting an inner shell from being filled. Their chemical properties are therefore similar and internal electronic transitions between the energy levels are possible and shielded by the outer shell. Thus, their fluorescence colors are in general largely insensitive to their chemical and structural environment. However, a person skilled in the art will be able to measure a few differences in the fineness of their fluorescence spectrum such as, for example, a variation in intensity of the various fluorescence lines or in the width of these lines depending on the nature of the crystalline field (R. Reisfeld et al., Journal of Luminescence, 2003, 102-103, 243-247). However, in general the position of these lines (i.e. the energy levels of these centers) will remain unchanged.
The fluorescence of silica glasses containing rare-earth ions has been studied. Thus, a number of authors have demonstrated a quenching effect by too high a concentration of fluorescence centers in silica glasses (K. Rosenhauer and F. Weidert, Glastech. Ber., 1938, 16, 51-57). Indeed, for neodymium oxide contents greater than 10% the glasses studied no longer fluoresce.
Moreover, it follows from the teaching of U.S. Pat. No. 2,097,275 that the presence of iron in a glass matrix, in an amount of 0.01% by weight or higher, could lead to the quenching of samarium fluorescence. It has also been demonstrated that, below this limit, there is an optimum intensity of samarium fluorescence as a function of the iron content.
However, U.S. Pat. No. 2,254,956 relates to a study in which a lead or bismuth synergy effect on the fluorescence intensity of cerium Ce3+ has been demonstrated. Thus, in aluminosilicate glasses, the fluorescence of Ce3+ between 334 and 480 nm, under excitation at a wavelength of 253.7 nm, is increased by 50% by introducing a few percent of lead oxide (3 wt % maximum), without the emission line width being affected thereby. Furthermore, the above patent indicates that if the material contains large amounts (30 wt %) of calcium fluoride, the intensity is further increased, as is the width of the emission band (303 nm to 480 nm).
Fluorescent glasses may be used in a variety of fields:                in the optics field, these materials are used as optical components (filters, optical fibers, etc.). U.S. Pat. No. 6,916,753 in particular describes a thulium-doped silica glass, the fluorescence of which lies at around 1400 nm: such a glass finds applications in the field of optical fibers. Likewise, U.S. Pat. No. 6,879,609 describes, in the case of optical fibers, a thulium-doped aluminosilicate glass, the excitation of which in the infrared (1060 nm) gives several fluorescence peaks in the visible according to an up-conversion photon emission process. Finally, U.S. Pat. No. 6,762,875 discloses a method for creating optical index variations for optical components using rare earths;        as glass diagnostic tools: pure glasses do not fluoresce as they do not absorb ultraviolet (UV) radiation. Thus, the characterization of matrices (appearance or disappearance of glassy zones depending on the heat treatment received) and the presence of certain impurities may be demonstrated by studying the fluorescence properties of the materials;        in the illumination field, their use however remains limited: it seems to be more effective to excite a film of crystalline phosphors deposited on a nonfluorescent glass rather than exciting a glass containing luminophore. However, certain compositions seem to give useful results. Thus, patent application EP 0 338 934-A1 discloses a composition based on Ce, Tb and Mn for obtaining white fluorescence under excitation by a low-pressure mercury lamp. The glass matrix used in this case is boron oxide (B2O3) or a boron oxide/silicon dioxide (B2O3/SiO2) mixture in which the SiO2 content is less than 20 mol %;        other types of fluorescent glass are also capable of finding various applications (illumination in mercury vapor lamps, display panels, decorative lighting, etc.). To give an example, U.S. Pat. No. 4,038,203 provides a number of different compositions for obtaining various colors in an alkaline phosphate glass activated by yttrium oxide. Thus, by doping this glass with europium oxide, pink fluorescence is obtained under excitation at a wavelength of 400, 460 or 530 nm. A green tint is produced by introducing terbium oxide. The color blue, obtained by doping with thallium, has however the major drawback of requiring the material to be excited at a wavelength of 250 nm, since no fluorescence can be obtained under excitation at 360 nm. Other colors, ranging from yellow to orange, may be obtained by codoping the material with terbium and europium in defined proportions; and        in the food packaging field, as described for example in international application WO 2006/20663, the use of rare-earth ions possibly combined with metal ions such as titanium ions makes it possible to obtain soda-lime glasses having defined optical (UV filter) properties, for the purpose of preserving products better, while still remaining transparent, or for the purpose of obtaining color effects (fluorescence or dichroism).        
Studying the prior art therefore shows that there are few examples of industrial application of fluorescent silica glasses for decorative use. Only a few anecdotal cases may be reported. Moreover, these are examples held by a few antique collectors, such as:                uranium glass or “vaseline glass”: in this case, the fluorescence obtained under UV illumination is green. However, industrialization of this type of glass for decorative usage is difficult because of the legislation regarding uranium. Moreover, this type of glass is yellow in color when observed under normal illumination, which does not necessarily correspond to the desired effect; and        manganese-doped glasses create an orange coloration under UV illumination. The fluorescence color seems to vary according to the composition of the glass and also seems to be of low intensity. Under ambient illumination, the glass is not colorless.        
The examples described in the prior art show the possibility of obtaining various fluorescence colors under UV illumination in various glass matrices. However, the type of UV used to reveal such fluorescence varies from UVA (between 320 and 400 nm) to UVB (290 to 320 nm). However, UVB is very hazardous for the human eye and precludes envisaging an actual application in the decorative field.
Finally, the studies in the prior art show that the composition of the glass matrix is strongly dependent on the intensity of the fluorescence obtained. Thus, iron, which is a commonly encountered impurity, alters the fluorescence of the material right from very low contents (<0.01%).
Thus, it appears that there is no glass for decorative use that currently meets all the following optical characteristics:                being transparent (colorless or colored) under illumination using a “white” source (sunlight, incandescent light, neon tube, halogen bulb, etc.), while still possibly having dichroic properties (in which the color varies according to the white light source used to illuminate the article);        being fluorescent under UVA illumination, preferably at a wavelength between 360 and 400 nm, the emission intensity being sufficient to be perceived by the human eye; and        being able to be produced within a wide fluorescence color palette, while still meeting, however, the two abovementioned characteristics: blue, yellow, green, orange, red and white.        